Recently, not only various new types of foods but also various new types of containers for the foods have been created. Therefore, now various types of foods contained in various types of containers are commercially available.
In the preparation of a food contained in a container, the food is sterilized before, during or after the container is filled with the food. The sterilizing procedure is usually carried out by heating the food at an elevated temperature over a considerably long period of time. This heating operation when applied to the food, sometimes causes the food per se to be spoiled, or adversely affects the taste and flavor of the food.
In order to eliminate the disadvantages of the above-mentioned heat-sterilizing method, it has been attempted to heat the food contained in the container at an extremely high temperature of, for example, 120.degree. to 140.degree. C., for a short time of several minutes by, for example, a so-called retort sterilizing method. This method is effective for shortening the time in which the food is exposed to the extremely high temperature. However, since the container which contains therein the food is also exposed to the extremely high temperature, it is necessary that the container per se be made of a thermally resistant material which is stable at the extremely high temperature. Usual paper and thermoplastic polymer materials are not suitable for the container due to the unsatisfactory thermal resistance thereof.
Also, it is known that microwaves are effective for sterilizing various foods. This sterilizing procedure can be effected at a relatively low temperature of 70.degree. to 105.degree. C. within a relatively short time of 90 to 180 seconds.
In the case where a food contained in a vessel is placed in a microwave-irradiating apparatus and microwaves are applied to the food to sterilize it, various volatile substances in the food are undesirably evaporated from the food. This evaporation of the volatile substances frequently causes the taste and flavor of the food to be changed.
In the case where a container capable of transmitting microwaves therethrough is filled with a food and, then, hermetically sealed, and thereafter, microwaves are applied to the food through the container, the evaporation of the volatile substances from the food can be prevented. However, in this case, the elevation in the temperature of the food results in expansion of the food. This expansion creates a large pressure in the container. Due to the expansion pressure created in the container, the container per se is bulged out and, sometimes, ruptures.
In order to prevent the rupture of the container, Japanese Patent Application Laying-open (Kokai) No. 52-68785 discloses a process in which a closed container containing a food material is placed in a hermetically closed box which is provided with an apparatus for emitting microwaves, and while the microwaves are applied to the container, the pressure in the closed box is controlled so as to equilibrate with the pressure in the container.
This process requires a special apparatus which causes the cost of the sterilizing procedure to be too high. Also, the process is very complicated and costly.
Under the above-mentioned circumstances, it has been recognized that an industrial application of the microwave sterilizing method to the food contained in a hermetically sealed container is very difficult and not practical.